Insulating glass (IG) units are preferred windows in residential and commercial buildings because they reduce energy transmittance through the window. As is appreciated by those skilled in the art, reducing energy transmittance through the IG unit reduces heat losses from the building interior during the winter, and reduces heat gain to the building interior during the summer. In general, the unit construction determines conductive and convective heat losses, and the absorption and reflection properties of the glass sheets of the IG unit control solar energy heat gain of the IG unit, an important parameter in commercial buildings. At the present time, a highly desirable commercial product is an IG unit with mid-range visible transmittance, low solar heat gain coefficient (SHGC), and neutral gray color. Of particular interest in the following discussion are the absorption, transmittance, reflectance, and color properties of the glass sheets of the IG unit. For a discussion of reducing conductive and convective heat loss of an IG unit reference can be made to U.S. Pat. No. 5,655,282, which patent is hereby incorporated by reference.
In general, increasing the amount of total iron content in the glass composition increases solar infrared (“IR”) energy absorption of the glass thereby reducing the IR transmittance. For example, a typical gray tinted float glass with the mid-range visible transmittance, e.g. 40-60% in the wavelength range of 380-770 nanometers of the electro-magnetic spectrum, contains greater than 0.3 wt % iron. Unfortunately, because the glass surface has a high emissivity, a significant portion of the absorbed energy flows to the building interior. Another drawback with increasing the total iron in the glass to reduce IR transmittance is that the glass heats when exposed to sunlight. As is appreciated by those skilled in the art, heating the glass by IR absorption sets up thermal stresses in the glass that can result in fracturing the glass sheet. A discussion of colored glass is provided in U.S. Pat. Nos. 3,723,142 and 6,998,362, UK Patent Application GB 2,252,973, and Japanese Patent 63-277537, which documents are hereby incorporated by reference.
Another technique to reduce the IR transmittance of the glass is to apply an IR reflective coating, e.g., of the type described in U.S. Pat. No. 4,610,771 and in U.S. patent application Ser. Nos. 10/428,481 and 10/912,718, which documents are hereby incorporated by reference. IR reflective coatings also typically have reduced surface emissivity, also referred to as “low e” coatings, thereby reducing heat flow through the window. A drawback with this technique to reduce IR absorption of the glass is that the coated glass may not have an aesthetically appealing color or desired level of visible transmittance. For example, and not limiting the discussion thereto, in general, IR reflective coatings on float glass deposited by magnetic sputtering vacuum deposition (“MSVD”) have a light green or yellow transmitted color. Using the MSVD IR coating with presently available glass substrates can have the following limitations. An IG unit having a coated clear glass sheet can have a high visible light transmittance, e.g. greater than 70%; an IG unit having a coated gray colored glass can have a low unit visible light transmittance, e.g. less than 36%; an IG unit having a coated blue/green or green colored glass is generally commercially considered too green in color, and an IG unit having a coated bronze colored glass is generally commercially considered too yellow in color.
Although, the presently available glasses coated with IR reflecting or low emissivity coatings are acceptable for some limited applications, they have limitations when used as one of the sheets of an IG unit. More particularly, several performance requirements of an IG unit include, but are not limited to, the IG unit having a low solar heat gain coefficient (“SHGC”), e.g. below 0.40, mid-range visible light transmittance of 40 to 60%, and a neutral gray color, e.g. a blue grey color. Using the presently available coated colored glass can provide an acceptable SHGC; however, if the color of the IG unit is not a neutral gray color, it has limited acceptability in the marketplace. One attempt to change the color of the coated glass to obtain an IG unit having a neutral gray color is to alter the color of the low e coating and apply the low e coating to clear glass. The color of the IG unit is more to the gray color; however, the SHGC of the IG unit is increased.
As can now be appreciated by those skilled in the art, it would be advantageous to provide a glass composition that can be used with presently available IR reflective coatings to provide a coated glass that can be used in an IG unit to provide an IG unit with a neutral blue gray color, a mid-range visible light transmittance, and a low SHGC.